Structure evolution and tetragonal phase stabilization mechanism of electrospun barium titanate nanofibers

In this research, barium titanate (BaTiO3) nanofibers were synthesized for the first time via the electrospinning process utilizing a conventional sol-gel precursor. Electrospinning is a fiber synthesis method that can generate nanofibers by applying a high electric field to a liquid precursor mixture. Fiber diameters ranged between 180 to 320 nm before heat treatment with average diameter of 273 nm. After proper heat treatment fibers crystallized and the diameters reduced to a range of 80 to 190 nm. The average diameter was 116 nm. The length of the heat treated nanofibers was in the order of a few hundred micrometers up to millimeters. X-ray analysis (Scherrer's formula) revealed 30 nm (+/- 6 nm) of crystallite size and a tetragonal structure was confirmed via {200} and {110} peak separations. Tetragonal distortion (c/a ratio) of ∼ 1.007 was obtained. Selected area electron diffraction patterns confirmed a perovskite structure. Furthermore, the advancement in heat treatment of the electrospun fibers yielded single crystalline BaTiO 3 nanofibers that were 40 nm in diameter and had lengths up to 0.6 microm. This is the first report of single crystalline electrospun nanofibers for a ternary oxide system.;Raman spectra obtained after crystallization matched well with the spectrum of bulk tetragonal BaTiO3. An extra peak at around 640 cm -1 was generated from planar defects such as (111) twins that can generate peaks located very similar to the hexagonal phase of barium titanate. An X-ray photoelectron spectroscopy study found that barium and titanium ions existed with a different oxidation states. This indicates defective and disordered structures during crystallization and supports intermediate phase formation and decomposition followed by BaTiO3 crystallization. Moreover, thermodynamic analyses were made to investigate factors stabilizing tetragonal structure with small critical crystallite size (30 nm, +/- 6 nm). The combined effect of strain and surface free energy was numerically calculated. Strain energy on crystallite composing BaTiO3 nanofibers was two orders of magnitude smaller (8.49 x 105 J/m2) than that observed from the cubic to tetragonal phase transition of a metal oxide system (ZrO2). Fiber deformation during the growth and single crystal structure of crystallites with negligible lattice misfits may contribute independently to lower strain energy. Williamson-Hall analysis confirmed strained BaTiO3 crystallites (by 0.255%). Surface energy of gas-solid interface expected to be between 0.00415 < gammag-s,tetra < 0.01 (J/m2). It is concluded that low strain energy of electrospun BaTiO3 attributed to stabilize tetragonal structure. This is an indication that electrospinning is an effective method for synthesizing one dimensional nano scale tetragonal BaTiO3 with proper strain energy release mechanism.